fix: search workspaces under ignored directory names

[engmohamedsalah]

Fixes #576 (reported by @Dhevenddra, with the root cause already nailed down in the report — thanks!).

Root cause

ripgrep resolves --glob patterns against paths relative to the process cwd, not the search root. grep passed the absolute workspace path while the agent process runs elsewhere, so the exclusion globs (!**/node_modules/** and the rest) matched the workspace's own ancestor directories. A workspace located under e.g. .../node_modules/ws excluded itself entirely — rg exited 1 with zero output.

Reproduced on macOS: same invocation from the workspace's own directory works, from any other cwd it returns nothing. (On Windows this fires for every pytest temp dir, since tmp_path lands under AppData.)

Fix

• Run rg with cwd anchored to the search base and . as the search target, so glob patterns can only match directory names inside the workspace
• rg now emits paths relative to that base (./a.py); _rel() resolves them against the search base before relativizing to the workspace root, keeping the documented relative-path output contract

Tests

• New: test_grep_works_when_workspace_sits_under_an_ignored_dir_name — workspace nested under node_modules, fails on current main (count: 0), passes with the fix
• Existing grep suite unchanged and green (15/15)

Note on ordering

This touches _parse_rg's signature, which #123 also rewrites (NUL parsing) — the changes compose cleanly but conflict textually. Suggested order: merge #123 first, then I rebase this onto it. Happy to do that as soon as it lands.